The functions of the cerebral cortex rely upon highly interconnected and dynamic microcircuits composed of two types of neurons: glutamatergic excitatory neurons that propagate signals through the various stages of processing, and GABAergic interneurons that regulate this information flow and sculpt cortical circuit dynamics. Signal processing in the cortex critically depends on the activity of specific interneuron subtypes. While the PV and SST interneuron populations have been well studied, this Program Project is focused on the 5HT3aR family of GABAergic interneurons whose prevalence, breadth and contributions to cortical function have been previously underestimated. 5HT3aR interneurons represent about 30% of the total interneuron population in the neocortex and are concentrated within the superficial associative layers, where they comprise the majority of interneurons. During development, 5HT3aR interneurons originate from the caudal ganglionic eminence (CGE), become positioned within the cortex late during the first postnatal week and contribute to both the function and plasticity of the cortex thereafter. Here we will investigate the roles of cortical 5HT3aR interneurons during development, as well as their function and plasticity within the somatosensory and auditory cortices. The Program Project will consist of three interrelated research projects and two cores (an Administrative Core and a Molecular and Transgenic Core) to support the work of the three projects. A focus of all three projects will be neocortical layer 1 (L1), the main cortical layer receiving contextual information. All neurons in L1 are interneurons of the 5HT3aR family. Project 1 (by Gordon Fishell), will elucidate the mechanisms that determine the development of the 5HT3aR interneuron population. It will investigate the genetic program that governs the differentiation of 5HT3aR interneuron precursors, their connectivity throughout development and the role of activity (with a particular focus on L-type Ca++ signaling) on their maturation in the cortex. Project 2 (by Bernardo Rudy) will advance our understanding of the role of 5HT3aR interneurons in cortical function by focusing on the interneuron subtypes that we have identified in L1 during the previous funding period. Specifically, Project 2 will investigate their input and output connectivity and their contributions to context-dependent sensory processing. Project 3 (by Robert Froemke) will examine the contributions of the same populations to both auditory processing and plasticity. This project will examine the role of L1 5HT3aR interneurons in awake behaving mice in an auditory recognition task and determine the requirement of cholinergic neuromodulation for this process. Together these projects will provide a comprehensive assessment of the 5HT3aR populations? development, plasticity and function.